This invention relates to a thermal barrier coating system having enhanced resistance to spallation, and, more particularly, to such a system wherein adhesion is enhanced and crack propagation is reduced by physical modification of at least one surface underlying the ceramic thermal barrier coating.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of hot gas turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gases then flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the exhaust gas temperature. However, the maximum temperature of the exhaust gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine. In current engines, the turbine vanes and blades are made of superalloys, and can operate at temperatures of up to about 1900-2100xc2x0 F. As used herein, the term superalloy includes high-temperature-resistant alloys based on nickel, cobalt, iron or combinations thereof.
Many approaches have been used to increase the operating temperature limit of the turbine blades and vanes, and other components of the engine that operate at high temperatures. The composition and processing of the materials themselves have been improved. Physical cooling techniques are used. In one widely used approach, internal cooling channels are provided within the components, and cool air is forced through the channels during engine operation.
To provide a further increase in the operating temperature limit, a thermal barrier coating system is applied to the turbine blade or turbine vane, which acts as a substrate. The thermal barrier coating system includes a ceramic thermal barrier coating that insulates the component from the hot exhaust gas, permitting the exhaust gas to be hotter than would otherwise be possible with the particular material and fabrication process of the component. Ceramic thermal barrier coatings usually do not adhere well directly to the superalloys used in the substrates, and therefore an additional layer called a bond coat is typically placed between the substrate and the thermal barrier coating. The bond coat improves the adhesion, and, depending upon its composition and processing, may also improve oxidation and corrosion resistance of the substrate.
The thermal barrier coating system must remain in place on the protected component to be useful. When the component is repeatedly heated and cooled, as occurs in the operating cycles of the gas turbine engine, thermally induced stresses and strains are produced and accumulate within the thermal barrier coating system due to the different thermal expansion coefficients of the ceramic thermal barrier coating and the metallic substrate to which it is applied. The bond coat helps to alleviate the buildup of stresses and strains, but they are present. The bond coat also improves the adhesion of the thermal barrier coating by improving the oxidation resistance of the substrate.
The most common mechanism of failure of the thermal barrier coating system is the spallation of the coating in local regions of the protected component. A crack is produced in the thermal barrier coating due to the accumulation of stresses and strains. The crack eventually propagates until a portion of the coating system flakes or chips away, this process being termed xe2x80x9cspallationxe2x80x9d. Such spallation failure usually occurs in patches. With the thermal barrier coating system locally removed, the underlying component is exposed to the hot exhaust gas temperatures, above which the unprotected component can not operate, and failure of the component quickly follows.
A number of techniques have been developed to reduce the tendency toward spallation failure of the thermal barrier coating. These techniques include optimization of compositions of the various layers, optimization of processing, adding new layers to the bond coat, and changes in design of the underlying components. The various approaches have been successful to varying degrees, but also involve drawbacks such as increased weight, constraints on design, and manufacturing complexity. Although progress has been made, the problem of spallation failure of thermal barrier coating systems remains.
There is therefore a need for an improved approach to improving the resistance to spallation failure of components protected by thermal barrier coating systems. The approach should be operable to extend the life of the protected component, and should be compatible with commercial production of engine components. The present invention fulfills this need, and further provides related advantages.
This invention provides an improved thermal barrier coating system and protected component. The approach of the invention increases the life of the component before the onset of spallation failure. In some instances, it may reduce the weight of the component, a particularly important consideration for a rotating component such as a turbine blade. The approach of the invention is compatible with other techniques to extend the life of the thermal barrier coating system, such as structural and compositional changes. The manufacturing of the blade requires an additional step, but this step is performed in an automated apparatus.
In accordance with the invention, an article protected by a thermal barrier coating system comprises at least a substrate having a substrate upper surface and a ceramic thermal barrier coating. An interfacial layer having an interfacial layer upper surface is optionally is applied between the substrate upper surface and the ceramic thermal barrier coating. A preselected, controllable pattern of three-dimensional features is applied to the substrate upper surface, the interfacial layer upper surface, or both.
Spallation failures occur when a crack is initiated in the thermal barrier coating system, typically in the interfacial layer or at one of the surfaces such as the aluminum oxide layer that grows on the interfacial layer and intermediate between it and the thermal barrier coating. The crack then propagates with increasing numbers of thermal cycles in a plane generally parallel to the surface of the substrate. Eventually, a small portion of the aluminum oxide, and any portion of the thermal barrier coating system located on it, is liberated from the substrate, leading to a failure of the thermal barrier coating system.
The present approach accepts the initiation of cracks in such a system. Rather than attempt to avoid such cracks entirely, the structure produced by the present technique seeks to arrest the propagation of the cracks by placing obstacles to crack propagation into the thermal barrier coating system. It will be understood, however, that the present approach may be used in conjunction with other techniques that seek to minimize crack initiation, the various techniques working together to prolong the life of the protected article.
The obstacles are three-dimensional features that deflect the crack tip and cause it to pass through phase boundaries which impede the progress of the crack. The result is that, while cracks may initiate, their propagation that leads to failure is impeded. The life of the thermal barrier coating system prior to spallation is thereby lengthened.
The three-dimensional features of the invention are present in a selectable, controllable pattern at the surface of the substrate or the interfacial layer. In the past, it has been known to have a high degree of surface roughness to improve the adherence of the thermal barrier coating to the interfacial layer. That roughness is produced by the mode of deposition or by chemical etching the surface. That prior approach is distinguished from that of the present invention by the selectability and controllability of the type of the three-dimensional features and the pattern of the features in the present invention. Selectability and controllability of the type and pattern of the features is important, as optimum crack-impeding geometries can be selected.
The three-dimensional features may be produced by many different methods. For example, a high energy beam such as a laser beam or an electron beam may be directed against the surface to which the pattern is to be applied; by moving the beam relative to the surface, a groove is created. Micromachining processes, in which one or more cutting tools are dragged over the surface, can provide an array of features. Abrasive flow machining is another form of micromachining. An engraving process in which selected portions of the surface are coated with etch-resistant material and the surface is then exposed to a suitable etchant also produces such three-dimensional features. The etch-resistant material may be applied by silk screening or lithography. Conventional photoengraving, in which photosensitive etch-resistant material is applied to the entire surface, locally sensitized by exposure to light passing through a mask, and chemically developed to form the preselected pattern, may be used. Where the surface to which the pattern is to be applied has an irregular shape, it may be more convenient to move the surface, previously coated with photosensitive etch-resistant material, under a stationary laser beam to achieve sequential exposure to the sensitizing light. There is an important distinction between etching in these engraving processes, where only selected portions of the surface are exposed to the etchant, and conventional chemical etching, in which the entire surface is exposed to the action of the etchant.
The preferred method for forming the three-dimensional features is a pulsed directed energy beam that ablates material from the surface against which it is directed. A pulsed excimer laser, operating in the ultraviolet range with pulses in the range of tens of nanoseconds duration, may be used. Such a laser is used to form the three-dimensional features by ablating material to be removed, in a way that has virtually no effect on the underlying material that is not removed. A clean pattern is formed, without introducing contamination, as sometimes occurs with conventional chemical etching, or cracks in the underlying material, as occurs with some other methods.
The three-dimensional features are preferably grooves. The grooves are preferably arranged parallel to each other in a pattern. With the excimer laser approach, arrays of grooves oriented at an angle to each other can be prepared. Other types of three-dimensional features such as dimples can also be used. The formation of the features by a directed energy beam permits great flexibility. One type of pattern or feature can be used in some areas, another type in another area, and none in yet other areas on the surface of the substrate.
The present invention provides an advance in the art of articles protected by thermal barrier coatings. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.